Due to the continual use of more and more electronics in vehicles, the Society of Automotive Engineers (SAE) encouraged the entire automotive industry to develop a standard data link, preferably a medium-speed (Class B) multiple access serial communications link. Already, SAE has established Recommended Practice J1850 (a set of technical requirements and parameters) and the industry has accepted a Class B data link as a J1850 as the recommended practice.
J1850 specifies use of symbols for communicating serial data over a communications bus. In variable pulse width modulation (VPWM) encoding, as used in the present invention, a symbol comprises a voltage logic level that extends for a period of time and then a voltage transition or edge occurs.
The amount of time between trip points of the previous edge and the current edge defines the meaning of the symbol. For example, a logic zero bit which can be either a short low of 64 .mu.s or a long high of 128 .mu.s represents the time between edges or transitions of the VPWM signal. J1850 specifies 3.875 volts as a nominal receiver trip point voltage parameter.
To minimize EMC problems during each transition of waveforms containing symbol information, waveshaping of the VPWM edges must take place. To satisfy RFI requirements of signals that reach the bus, each edge must have a certain slope and corner shape.
Within a transceiver, problems result in trying to maintain consistent transmitted trip points. Since the trip point of the previous edge provides the point of reference for the current edge, a problem occurs if the trip points of the transitions occur at different times.
J1850 specifies that communication buses may be two wires routed adjacently or twisted pairs dispersed throughout the network.
In prior systems, reshaping VPWM pulses to satisfy RFI requirements occur but usually without considering maintaining consistent transmitted pulse trip-points. Probably the prior system designers focus more attention to problems of voltage offset between nodes rather than transmitting clean pulses.
In an effort to minimize distortion of symbol time spans, a search took place to find other schemes which compensated for symbol time span distortion. This search resulted in the present invention which establishes consistent trip points along the edges of pulse signals placed on a communications data line.